Intracorporeal gas exchange devices, systems and methods

ABSTRACT

A system for intracorporeal gas exchange includes a flexible, rotatable shaft; a plurality of axially spaced agitation mechanisms positioned on the rotatable shaft, such that the rotatable shaft can flex between the axially spaced agitation mechanisms; a plurality of hollow gas permeable fibers adapted to permit diffusion of gas between intracorporeal fluid and an interior of the hollow fibers. The plurality of hollow fibers is positioned radially outward from the agitation mechanisms. A blood contacting medical system includes at least one seal, and a purge system via which a flushing fluid is introduced under pressure over a blood-side face of the seal and caused to flow through a space on the blood side of the seal. The system can further include a rotatable member. The space on the blood side of the seal can be in fluid connection with an annular space adjacent to a rotating element operatively connected to the rotatable member.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims benefit of U.S. Provisional PatentApplication Ser. No. 61/026,735, filed Feb. 7, 2008, the disclosure ofwhich is incorporated herein by reference.

GOVERNMENTAL INTEREST

This invention was made with government support under grant HL070051awarded by the National Institute of Health: National Heart, Lung andBlood Institute. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to intracorporeal gas exchange devices,systems and methods and, particularly, to devices, systems and method toeffect gas exchange within a patient's vascular structure.

The following information is provided to assist the reader to understandthe invention disclosed below and the environment in which it willtypically be used. The terms used herein are not intended to be limitedto any particular narrow interpretation unless clearly stated otherwisein this document. References set forth herein may facilitateunderstanding of the present invention or the background of the presentinvention. The disclosure of all references cited herein areincorporated by reference.

Lung disease remains one of the major healthcare problems present in theUnited States today. Two significant contributors to lung disease areacute respiratory distress syndrome (ARDS), and chronic obstructivepulmonary disease (COPD).

Acute respiratory distress syndrome is defined as a severe syndrome ofinflammation and increased vascular permeability that is associated witha constellation of clinical, radiologic, and physiologic abnormalities.It is a non-cardiogenic, acute respiratory complication characterized bya profound reduction in systemic oxygenation or ventilation, with anin-hospital mortality rate of 38.5%. ARDS develops as a result of injuryto the endothelium and epithelial layers of the alveolar membrane fromstimuli such as sepsis, infection or trauma. The breakdown of thecapillary-alveolar barrier leads to alveolar flooding and an eventualdeterioration of gas exchange capability.

In contrast to ARDS, chronic obstructive pulmonary disease is apersistent, irreversible condition that slowly progresses over time.COPD refers to the existence or co-existence of chronic bronchitis andemphysema and is characterized by obstructed airways, enlarged airspaces and destruction of lung parenchyma, occlusion of small airways,and reduced lung elasticity. When compared to normal, healthy lungfunction, patients with advanced stages of COPD can experience 10 to 20times the work necessary to facilitate breathing.

Hospital treatment for ARDS, acute exacerbations of COPD, and advancedCOPD exist in three broad generalizations: pharmacological, mechanicalventilation and application of an extracorporeal membrane device(artificial lung). The treatment modalities depend on the severity ofthe disease as well as patient response to prior therapies. As indicatedby the high mortality rates for the lung conditions, however, eachtherapy has associated limitations which can interrupt patient dischargefrom the hospital and/or recovery.

The least invasive and first line of defense in treating lung failure isto administer drugs that may improve the ailing condition. Multiplepharmacologic strategies have been investigated since the 1960's, butdespite therapeutic benefits, none of the investigated treatments havedemonstrated an ability to improve patient survival.

Mechanical ventilation is the most common therapy and serves to maintainrespiratory function by rhythmically inducing a controlled flow of airinto the lungs. In healthy persons, normal breathing consists ofcontracting the diaphragm to distend the lungs and create a negativepressure from atmosphere to lungs forcing fresh air in. Following oxygenand carbon dioxide gas exchange, the diaphragm relaxes compressing thelungs and forcing expiratory air to the external environment. Mechanicalventilation creates this effect but in an opposing manner; fresh air isdriven into the lungs by positive pressure and expiratory air is pumpedout of the lungs by a negative pressure.

Mechanical ventilation treatment is associated with multipleshortcomings termed ventilator-induced lung injury, or VILI. VILI coversa range of detrimental insults to the lung that can postpone recovery,cause unfavorable outcomes, or even intensify preexisting injury.

The third clinical therapy being administered utilizes a membraneoxygenator and accompanying flow circuit. The treatment is denoted asextracorporeal membrane life support (ECLS), often times referred to asextracorporeal membrane oxygenation (ECMO). An ECLS device processespatient blood by adding oxygen and removing carbon dioxide through fibermembrane technology, replicating the natural gas exchange function ofthe lungs.

ECLS is employed under circumstances of severe, reversible respiratoryfailure or to patients responding adversely to all advanced modes ofmechanical ventilation. Operation of the circuit relies on a pump todraw blood from the vena cava, transport it through the membraneoxygenator, and return the blood either to the right atrium (venovenousbypass) or aorta (venoarterial bypass). Patients still receivemechanical ventilation while on ECLS, however settings are reduced tominimize VILI as a result of the ability of the oxygenator to exchangeblood gases. With less required work from the lungs, ECLS permitsphysiological complications to abate and the therapy can be applied forweeks barring complications.

Limitations of ECLS primarily arise from external circuitry andartificial blood contacting surfaces. To avert thrombosis within thecircuit, patient blood is continuously anticoagulated and bleeding is amajor risk whether internal (intercranial) or from cannula dislodgement.Patients are paralyzed and/or heavily sedated to minimize movementcausing dislodgement, which creates a high risk scenario for decubitisulcers. Also, the continuous exposure of the blood to artificialsurfaces causes platelets to adhere and/or alter function(thrombocytopenia) requiring the patient to receive multiple platelettransfusions. In addition, the ECLS circuit must be constantly monitoredfor mechanical failures such as tubing degradation, oxygenator or pumpfailure, and presence of gaseous emboli or clot formation. Other notedcomplications include sepsis and renal failure. Finally, ECLS requires amultidisciplinary team to provide care. Staffing and overall cost of theprocedure, as well as restriction to major medical centers are furtherlimitations to providing this therapy.

Recently, there have been attempts to develop intracorporeal artificiallung devices. Percutaneous, intravascular respiratory support therapy,which refers to treatments employing a gas exchange device or anartificial lung device within the vasculature of a patient to supplementlung function, can be used as an additional and/or alternative approachto treating ARDS and acute exacerbations of COPD. Such devices typicallyuse hollow fiber membrane or HFM technology to achieve gas exchange,oxygenating blood and removing carbon dioxide. In most situations,carbon dioxide removal is a primary goal of intravascular devices sincesufficient oxygenation levels can be attained in the clinical settingthrough nasal oxygen or specific ventilation modes.

The ability of intravascular respiratory support devices to facilitatecarbon dioxide removal from the circulation provides an advantage oversole mechanical ventilation strategies. By regulating hypercapnia (orelevated carbon dioxide levels in the blood), intravascular respiratorytherapy can allow ventilation at lower tidal volumes and pressures andthereby eliminate the deleterious effects that often develop withmechanical ventilation. Decrease in mechanical ventilation intensity tosupport patients has been demonstrated to improve mortality rates. Inaddition, the diseased lung tissue experiences a lower workload sincethe device itself is performing partial respiratory function. Thereduced workload allows the injured tissue to rest and may improvetissue recovery.

ECLS is able to regulate hypercapnia but is associated with a number ofcomplications resulting from blood circulating outside the body.Utilizing an intracorporeal/intravascular device eliminates externalcircuitry, thereby lessening the risks of thrombocytopenia andactivation of complement resulting from artificial surfaces. Lessartificial surface can also result in lower anticoagulation levelsthereby decreasing bleeding risks. Overall implementation of theintravascular respiratory therapy is easier than ECLS making it lessdemanding on hospital resources, less expensive, and potentiallyavailable in more hospitals.

Factors regulating gas exchange in the intravascular devices include gaspartial pressures in both blood and fiber lumens, total HFM surfacearea, fiber bundle geometry and relative velocity at which the bloodpasses by the fiber surfaces. A concentration boundary layer that formsnear the external walls of the individual fibers can result insignificant resistance to diffusion that limits gas exchange. Theboundary layer forms as a result of blood flow patterns around the fiberwalls.

Existing intravascular devices can be categorized as either passive oractive by the means in which boundary layer reduction is approached.Passive devices rely on blood flowrates and fiber bundle geometry to mixflow patterns and disrupt layer formation near stationary fibers. Activedevices implement motion to the fiber membranes and or to the bloodsubjacent to the fiber membranes to disrupt boundary layer formation. Ingeneral, to increase gas exchange, additional surface area can beprovided to boost overall diffusion and/or boundary layers can bereduced to decrease mass transfer resistance. High relative bloodvelocity to fibers facilitates boundary layer reduction.

As clear to those skilled in the art, a relatively small size isbeneficial for intracorporeal/intravascular respiratory assist devices.Such devices can, for example, be inserted into a patient's vascularstructure in the manner of a catheter. Reducing insertion size of thedevice is desirable to prevent tissue damage and facilitate placement.However, limiting the size of the intravascular device also results inlimitations upon surface area of the gas exchange membrane system,thereby limiting gas exchange. Further, the amount of motion that can beimparted to blood and/or fibers to increase gas exchange efficiency islimited by potential damage to surrounding tissue and to the blood.Previously studied intracorporeal/intravascular devices have met withonly limited success as a result of these limitations.

It thus remains desirable to develop improved intracorporeal gasexchange devices, systems and methods that are safe, effective, and/oramenable to easy insertion.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a system forintracorporeal gas exchange including a flexible, rotatable shaft; aplurality of axially spaced agitation mechanisms positioned on therotatable shaft, such that the rotatable shaft can flex (more readily)between the axially spaced agitation mechanisms; a plurality of hollowgas permeable fibers adapted to permit diffusion of gas betweenintracorporeal fluid and an interior of the hollow fibers. The pluralityof hollow fibers is positioned radially outward from the agitationmechanisms.

The system can further include a support member positioned radiallyoutward from the agitation mechanisms. The plurality of hollow fiberscan, for example, be positioned radially outward of and adjacent to thesupport member.

Each of the plurality of agitation mechanisms can, for example, includeat least one radially outward extending vane.

The system can further include a medial manifold and a distal manifold.A first end of each of the plurality of hollow fibers can, for example,be in fluid connection with the medial manifold. A second end of each ofthe plurality of hollow fibers can be in fluid connection with thedistal manifold. The device can further include a first fluid path forflow of an inlet gas into the medial manifold to flow into the firstends of the hollow fibers and a second fluid path in fluid connectionwith the distal manifold into which outlet gas exiting the second endsof the hollow fibers can flow to exit the device.

The system can also include an extracorporeal proximal manifoldincluding an inlet in fluid connection with the first fluid path and anoutlet in fluid connection with the second fluid path.

At least a portion of the second fluid path can be within the flexiblerotatable shaft.

The system can also include a drive shaft and a drive system to impartrotation to the drive shaft. The drive shaft can, for example, passthrough the proximal manifold and through the second fluid path to be inoperative connection with the rotating shaft.

The system can further include a vacuum in fluid connection with theoutlet of the proximal manifold.

In several embodiments, at least one of the medial manifold and thedistal manifold includes at least one seal, and the device furtherincludes a purge system via which a flushing fluid is introduced underpressure over a blood-side face of the seal and caused to flow through aspace on the blood side of the seal. The space on the blood side of theseal can, for example, be in fluid connection with an annular spaceadjacent to a rotating element in operative connection with therotatable shaft.

The medial manifold can, for example, include at least a first seal, andthe distal manifold can include at least a second seal. The purge systemcan introduce flushing fluid under pressure over a blood-side face ofthe first seal and cause flushing to flow through a space on the bloodside of the first seal. The purge system can also introduce flushingfluid under pressure over a blood-side face of the second seal and causeflushing fluid to flow through a space on the blood side of the secondseal.

The space on the blood side of the first seal can be in fluid connectionwith an annular space adjacent to a first rotating element operativelyconnected to the rotatable shaft, and the space on the blood side of thesecond seal is an annular space adjacent to a second rotating elementoperatively connected to the rotatable shaft.

The purge system can further include a first conduit to deliver flushingfluid to the medial manifold and a second conduit to delivery flushingfluid to the distal manifold.

In several embodiments, the drive system and drive shaft are adapted torotate the rotating shaft at a rate of at least 10,000 to 20,000 RPM.

The distal manifold, the hollow fibers and the medial manifold can, forexample, be adapted to be inserted intravascularly and can have amaximum outer diameter of no more than 33 French or no more than 25French.

In several embodiments, the device comprises immobilized carbonicanhydrase on or in the vicinity of at least a portion of a first surfaceof the hollow fibers.

The system can further include immobilized carbonic anhydrase on or inthe vicinity of at least a portion of a first surface of the hollowfibers such that the immobilized carbonic anhydrase comes into contactwith the fluid. The first surface can, for example, exhibit carbonicanhydrase activity of at least 20% of maximum theoretical activity ofthe first surface of the hollow fibers based on monolayer surfacecoverage of carbonic anhydrase in the case that the carbonic anhydraseis immobilized on the first surface of the hollow fibers.

In another aspect, the present invention provides a system forintracorporeal gas exchange including: a rotatable shaft; at least oneagitation mechanism positioned on the rotatable shaft; a plurality ofhollow gas permeable fibers adapted to permit diffusion of gas betweenintracorporeal fluid and an interior of the hollow fibers, the pluralityof hollow fibers being positioned radially outward from the agitationmechanism; at least one seal, and a purge system via which a flushingfluid is introduced under pressure over a blood-side face of the sealand caused to flow through a space on the blood side of the seal. Thespace on the blood side of the seal can, for example, be in fluidconnection with an annular space adjacent to a first rotating elementoperatively connected to the rotatable shaft.

In a further aspect, the present invention provides a blood contactingmedical system including at least one seal, and a purge system via whicha flushing fluid is introduced under pressure over a blood-side face ofthe seal and caused to flow through a space on the blood side of theseal. The system can further include a rotatable member. In severalembodiments, the space on the blood side of the seal is in fluidconnection with an annular space adjacent to a rotating elementoperatively connected to the rotatable member.

The devices of the present invention provide significant advantages overpreviously studied intracorporeal/intravascular gas exchange devices. Tobe effective and obtain clinical acceptance, such devices are preferablysafe in operation, effective, and amenable to easy insertion. Ingeneral, these criteria require an intravascular device to be relativelysmall in size. In general, increased gas exchange efficiencies (both CO₂removal and O₂ exchange) of the devices and systems of the presentinvention, allow the devices and systems to be fabricated withrelatively small outer diameters. The devices of the present inventioncan, for example, be used for respiratory support of patients with acuteor acute on chronic lung failure. In many situations, the devices willbe used for short periods of time (for example, two weeks or less) inclinical setting. In most cases, a primary performance goal will be CO₂removal as sufficient oxygenation levels can usually be attained in aclinical setting through nasal oxygen or specific lung protectiveventilation modes. The devices of the present invention can, forexample, achieve CO₂ removal rates of at least 30 ml/min, at least 50ml/min, and even at least 75 ml/min under normocapnia conditions. Thedevices of the present invention can facilitate ventilatory lungprotective strategies by regulating hypercapnia conditions. Removalrates can increase under permissive hypercapnia.

The present invention, along with the attributes and attendantadvantages thereof, will best be appreciated and understood in view ofthe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates insertion of an embodiment of an intravascular deviceof the present invention through the femoral vein for placement in thevena cava.

FIG. 2A illustrates a photomicrograph of a fiber wall of a hollow fibermembrane.

FIG. 2B illustrates a perspective view of fiber ends of a hollow fibermembrane bundle.

FIG. 3 illustrates a model of gas exchange (oxygenation of and carbondioxide removal from) blood via a hollow fiber membrane.

FIG. 4 illustrates a perspective view of an embodiment of anintravascular device of the present invention.

FIG. 5A illustrates an enlarged view of an impeller unit of the deviceof FIG. 4A positioned upon the impeller shaft of the device of FIG. 4.

FIG. 5B illustrates a cross-sectional view of the device of FIG. 4.

FIG. 5C illustrates an enlarged cross-sectional view of the device ofFIG. 4.

FIG. 5D illustrates an enlarged cross-sectional view of a proximalmanifold of the device of FIG. 4.

FIG. 6A illustrates a perspective view of the medial manifold of thedevice of FIG. 4.

FIG. 6B illustrates an enlarged cross-sectional view of the medialmanifold.

FIG. 6C illustrates another enlarged cross-sectional view of the medialmanifold.

FIG. 6D illustrates another enlarged cross-sectional view of the medialmanifold.

FIG. 7A illustrates a perspective view of the distal manifold of thedevice of FIG. 4.

FIG. 7B illustrates an enlarged cross-sectional view of the distalmanifold.

FIG. 8A(1) illustrates a “2-flat” impeller unit geometry studied in thepresent invention.

FIG. 8A(2) illustrates a “4-flat” impeller unit geometry studied in thepresent invention.

FIG. 8A(3) illustrates a “sawtooth” impeller unit geometry studied inthe present invention.

FIG. 8A(4) illustrates a “paddle” impeller unit geometry studied in thepresent invention.

FIG. 8A(5) illustrates a “centrifugal” impeller unit geometry studied inthe present invention.

FIG. 8A(6) illustrates a “cone” impeller unit geometry studied in thepresent invention.

FIG. 8A(7) illustrates a “RAX” impeller unit geometry studied in thepresent invention.

FIG. 8A(8) illustrates an “axial” impeller unit geometry studied in thepresent invention.

FIG. 8B illustrates an enlarged perspective view of a sawtoothembodiment of an impeller unit of the present invention.

FIG. 8C illustrates an enlarged front view of a sawtooth embodiment ofan impeller unit of the present invention.

FIG. 9A illustrates an experimental setup for studies of gas exchangeefficiency of devices of the present invention.

FIG. 9B illustrates an experimental setup for studies of fatigue testingin bovine blood to assess performance of the devices of the presentinvention.

FIG. 9C illustrates a study of carbon dioxide exchange rate per unitarea for four impeller geometries of the present invention as a functionof impeller rotation rate.

FIG. 9D illustrates a study of oxygen exchange rate per unit area forfour impeller geometries of the present invention as a function ofimpeller rotation rate.

FIG. 9E(1) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(2) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(3) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(4) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(5) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(6) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(7) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(8) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9E(9) illustrates a graph of experimental results for carbondioxide removal as a function of impeller rotation speed in deionizedwater and in a viscous fluid for the indicated impeller unit geometry.

FIG. 9F(1) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(2) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(3) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(4) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(5) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(6) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(7) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(8) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 9F(9) illustrates a graph of experimental results for carbondioxide removal as a function of the square root of impeller rotationspeed for the indicated impeller unit geometry.

FIG. 10 illustrates a graph of motor current versus elapsed time for acontrol and for a device of the present invention including salinepurge.

FIG. 11 illustrates a graphical representation of carbon dioxide removalrate per unit area in an animal study for three impeller geometries ofthe present invention.

FIG. 12 illustrates carbon dioxide removal rate per unit area in ananimal study for three impeller geometries of the present invention as afunction of impeller rotation rate. For each device results are shownfor two different positions within the animal vena cava, one of whichhas the device spanning the right atrium

FIG. 13 illustrates blood plasma free hemoglobin (PFHB) levels as afunction of time in two animal studies for two impeller geometries ofthe present invention.

FIG. 14A illustrates a comparison of the operation of a standard hollowfiber membrane with a membrane of the present invention in whichdiffusion is facilitated by carbonic anhydrase immobilized on or in thevicinity of the membrane surface in contact with the blood plasma (whichoccurs because the immobilized enzyme generates diffusion of bicarbonateion (an alternative form of CO₂), which is a substrate of the enzyme,towards the membrane, at which the bicarbonate ion is converted to CO₂,which then diffuses into and across the membrane).

FIG. 14B illustrates a schematic representation of an RFGD process forimmobilization of carbonic anhydrase on a polymeric material.

FIG. 15A illustrates a study of the effect of fluid viscosity onrequired motor torque.

FIG. 15B illustrates a tortuous test section used in studies of thepresent invention.

FIG. 15C illustrates a study of measured torque levels of tubing samplesin tortuous configurations.

FIG. 16A illustrates a schematic representation of concentric rotatingcylinders.

FIG. 16B illustrates schematically a side view an embodiment of animpeller catheter external sleeve.

FIG. 16C illustrates schematically a front view an embodiment of animpeller catheter external sleeve.

FIG. 17 illustrates a study of hemolysis generation in several animalimplant studies.

DETAILED DESCRIPTION OF THE INVENTION

In several embodiments, the present invention provides intracorporealrespiratory assist devices that can, for example, be inserted in apercutaneous manner similar to insertion of a catheter which areoperable to at least partially support native lung function in patientswith, for example, acute respiratory distress syndrome and/or acuteexacerbations of chronic obstructive pulmonary disease. Intravasculardevices of the present invention are sometimes referred to herein ascatheters. As described above, primary current clinical therapies(including pharmacotherapy, mechanical ventilation, and ECLS) areassociated with patient injury and high mortality rates. The devices ofthe present invention can be used in combination with or as analternative to such clinical therapies to reduce patient injury andmortality.

Although the devices, systems and methods of the present invention arediscussed primarily herein in connection with oxygenation of and removalof carbon dioxide from blood, one skilled in the art appreciates thatthe devices, systems and methods of the present invention can be usedfor many other types of gas exchange (for example, for delivery ofanesthesia).

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the content clearly dictatesotherwise. Thus, for example, (unless clearly indicated otherwise)reference to “an impeller unit” includes a plurality of impeller unitsand equivalents thereof known to those skilled in the art, and so forth,and reference to “the impeller unit”, is a reference to one or more suchimpeller units and equivalents thereof known to those skilled in theart, and so forth.

The devices of the present invention can, for example, be insertedthrough the femoral vein for placement in the vena cava in the manner ofa catheter to actively process venous blood as illustrated in FIG. 1. Inseveral embodiments, the devices of the present invention include a gasexchange membrane system including, for example, a plurality of hollowfiber membranes (HFM), sometime referred to herein as a HFM bundle, thatsupplement oxygenation and carbon dioxide removal through diffusionalprocesses as illustrated in FIGS. 2A through 3.

In general, the intracorporeal devices and systems of the presentinvention can be of reduced size when compared to previous devicesdesigned for intravascular respiratory assist, which facilitatespercutaneous insertion. Reduction in the size, however, can limitavailable membrane (for example, HFM) surface area. In a number ofembodiments, efficiency in removal of CO₂ per unit membrane surface areais improved to offset limits in available surface area. Furthermore,oxygen (O₂) transfer rates per unit surface are also improved ascompared to currently available intracorporeal and other devices. Inseveral embodiments of the devices of the present invention, agitationelements or mechanisms such as rotating elements, impellers or rotors(sometimes referred to herein collectively as impellers) are positionedwithin a stationary HFM bundle to impel flow or create forced flowwithin and through the HFM bundle to increase efficiency of gas exchangeper unit surface area. Internalization of rotational components withinthe HFM bundle of the devices of the present invention assists inprotecting the surrounding tissue from damage.

Devices of the present invention were evaluated in vitro and in vivo toassess design and performance Acceptable hemolysis levels were observedwith high-speed rotational impellers within HFM bundles. Standard gasexchange characterization tests in, for example, deionized or DI watershowed an increase in CO₂ removal efficiency and O₂ transfer/exchange ascompared to previously disclosed percutaneous respiratory assist devicesor oxygenators. Several embodiments of devices of the present inventionwere evaluated in a bovine model and an average efficiency of 513±20 mlCO₂/min/m² was attained at 20,000 RPM.

As described above, membrane oxygenators employ hollow fiber membrane(HFM) technology to achieve gas transfer. The HFMs can, for example, bemade from polymers such as polymethylpentane or polypropylene that areextruded or otherwise formed into microporous tubes. The miniature tubescan further be arranged into bundles that are used in a number ofcurrently available oxygenators. FIGS. 2A and 2B displays a magnifiedview of HFMs. Wall porosity enables the membranes to exchange oxygen andcarbon dioxide gases while blood flows external to the fibers. The wallsof the fibers are hydrophobic and prevent fluid from leaking into thelumens.

The process driving gas exchange in HFM is diffusion. Oxygenators can,for example, be setup to process venous blood that is high in carbondioxide (CO₂) concentration and low in O₂ concentration. Gas rich inoxygen or pure oxygen (O₂) gas is passed through the HFM lumens at apressure (for example, pO₂≈740 mmHg) that is greater than the gastension/partial pressure of oxygen in the blood. A transmembraneconcentration gradient is thus created between venous blood (pO₂=38mmHg, pCO₂=45 mmHg) and fiber lumen. The high concentration of CO₂ inthe blood favors diffusion into the low concentration side within theHFM lumen, and conversely the high concentration of O₂ in the HFM favorsdiffusion into the low concentration blood-side. A schematicrepresenting CO₂ diffusion through a hollow fiber membrane is shown inFIG. 3. Blood passing through the HFM bundle is thus supplemented withoxygen and removed of carbon dioxide in a fashion similar to the naturallung.

During insertion, the devices or catheters of the present invention can,for example, extend through the femoral vein into the vena cava to spanthe right atrium of the heart. FIG. 1 illustrates insertion of a device10 of the present invention within the vena cava prior to reaching aposition spanning the right atrium. A position spanning the right atriumallows the device to process all venous blood returning from the majorvessels of the body. Furthermore, flow from the vena cava into the rightatrium creates an additional blood mixing component since cross-flowwill occur. This mixing regime may influence boundary layer formation.

Pure oxygen gas can, for example, be shuttled through the HFM bundleunder a vacuum so that O₂ transport occurs by diffusion only. The vacuumassists in ensuring that oxygen sweep gas is not bubbled through thepores of the HFM into the blood as a result of a high transmembranepressure gradient. In several embodiments described in further detailbelow, manifolds at the proximal and distal ends of the fiber bundlecollectively organize the fibers to permit sweep gas flow in parallelthrough the bundle. Conduits in fluid connection with the manifoldedbundle create a single, continuous flow path for sweep gas. Flowratethrough the device can, for example, be maintained approximately 40-60times greater than the CO₂ exchange rate to maximize efficiency. See,for example, Federspiel W J, Hattler B G: Sweep gas flowrate and CO₂exchange in artificial lungs. Artif Organs 20: 1050-1052, 1996, thedisclosure of which is incorporated herein by reference. Lower flowratescan lead to CO₂ accumulation within the HFMs which reduces the gasconcentration driving gradient responsible for diffusion. The flowraterange set forth above assists in assuring that gas transfer is maximizedand not dependent upon sweep gas flowrate.

Preferably, the devices of the present invention are operable withoutcausing harmful effects to the blood. In that regard, actively mixingthe blood can cause hemolysis if erythrocytes experience excessive shearstresses and exposure times. In several embodiments, the devices of thepresent invention were designed (but not optimized) to minimizehemolysis (for example, as determined by maintaining acceptable bloodplasma free hemoglobin (PFHB) levels).

As described above, in reducing the insertion size of the catheter,there is a resultant loss in HFM surface area. In the devices of thepresent invention, a novel active mixing approach is incorporated toreduce concentration boundary layers to counterbalance the loss of HFMsurface area and provide acceptable overall gas exchange. In severalembodiments, at least one rotating impeller unit was positioned withinthe HFM bundle to distribute fluid through the bundle. The impellerunits were designed so that the HFM bundle would remain stationary andenclose the rotating components.

Gas exchange tests were first performed to evaluate gas exchangeefficiency. The tests were further subdivided to analyze the effects ofimpeller geometry on gas exchange in both the standard deionized (DI)water solution and a blood representative viscous solution. Tortuousloop tests were implemented in early stages to ensure mechanicaloperation in severe bending environments. Fatigue tests were conductedin bovine blood to assess hemolysis. The devices of the presentinvention were also tested in four acute calf studies to examine gasexchange between different impeller geometries.

One embodiment of a device 10 of the present invention is illustrated inFIGS. 4 through 7B. Device 10 includes a plurality of spaced internalimpeller units or elements 100 capable of rotating at, for example,20,000 RPM to impart fluid motion and to force fluid radially outward.In the embodiment as illustrated, for example, in FIGS. 4 and 5A,impeller units 100 included four flat vanes 110 extending radiallyoutward from a common cylindrical base 104 which was positionable onflexible impeller shaft 120. The impeller system can also be describedas a single agitation or impeller system including a plurality of groupsof vanes that are axially spaced on shaft 120. An HFM bundle 200, asdescribed above, enclosed rotating impeller units 100. In theillustrated embodiment, HFM fabric 200 was formed around a support 300in the form of, for example, a stainless steel spring or helical coil.In several devices 10 studied in the present invention, a 5 mm diametercavity existed within stent or spring 300 to create a hollow bundlemodule for impeller units 100 to reside. Spring 300 served as both abackbone or support to support HFM fabric/HFM bundle 300 and as abarrier to shield HFM bundle 300 from rotating vanes 110 of impeller,even in tortuous configurations. Support 300 was secured to a medialmanifold 400 and a distal manifold 500 and stretched to provide amplespacing for fluid passage. As illustrated in, for example, FIG. 5B, inseveral studied embodiments, HFM bundle was approximately 30 cm inlength and 12 to 14 (14 in the illustrated embodiment) impeller units100 were positioned along shaft 120 with a spacing of approximately 0.5inches therebetween. In several studied embodiments of device 10,CELGARD® ARX30-240 hollow fiber membrane fabric (Celgard, Inc., ofCharlotte, N.C.) was used as fiber bundle 200. Characteristics of theindividual HFMs and the HFM bundle studied are set forth in Table 1.

TABLE 1 ARX30-240 HFM Material Polypropylene Outer Diameter (μm) 300Inner Diameter (μm) 240 Average Pore Size (μm) 0.03 Porosity (%) 40Fibers per Inch (FPI) 54 HFM Bundle Bundle Length (cm) 30 Outer Diameter(mm) 8 # of Fibers 250 Fiber Layers 4 Wefts none removed Surface Area(m²) 0.07

Hollow fiber membranes in artificial lungs (blood oxygenators orrespiratory assist devices) are typically made using polyolefinpolymers, with polypropylene, polyethylene and polymethylpentene beingcommonly used materials. The hollow fiber membranes are created byextrusion or other manufacturing processes to create a microporous fiberwall with sub-micron sized pores spanning the walls. These microporousfibers are adequate in commercial blood oxygenators for short term bloodcontact. In respiratory assist devices and artificial lungs required forlonger-term blood contact (>6 hours), the microporous fibers can leakplasma into the fiber lumens and create a problem known as plasmawetting. To prevent or retard plasma wetting, composite or asymmetricmicroporous hollow fiber membranes can be used in the devices andsystems of the present invention. A composite HFM is a standardmicroporous hollow fiber membrane on which a thin coating of anonporous, permeable polymer is applied. As the nonporous coating shouldbe very gas permeable, the coating is often made from a highly gaspermeable polymer such as siloxane polymers. An asymmetric microporoushollow fiber membrane is one in which the fiber wall is fabricated sothat the porosity varies across the wall and goes to zero at the fibersurface. The pores are then essentially sealed by the same gas permeablematerial from which the fiber is made.

Studied devices 10 were designed so that impeller units 100 could beeasily interchanged to rapidly evaluate different impeller geometries.Impeller units 100 were mounted directly to impeller shaft 120 viaadhesive. Fiber bundle 200 was constructed from 300 μm OD fibers 210(see FIG. 1) in fabric form, with fibers 210 potted into medial manifold400 and distal manifold 500. In that regard, epoxy was applied to fiberbundle 200 to fix manifolds 400 and 500 to bundle 200.

Distal manifold 500, medial manifold 400 and a proximal manifold 600operated, in part, to separate gas pathways in device 10. Proximalmanifold 600 was used to introduce, for example, oxygen sweep gas via aninlet port 610 and also included an outlet port 620 for exhaust gasremoval. Oxygen gas was introduced into inlet port 610 of proximalmanifold 600 and directed through HFM bundle 200. Distal manifold 500redirected exhaust gas back through impeller tubing 120 towards proximaloutlet port 620. Proximal manifold 600 also operated to couple a devicedrive line 700 to a drive system such as a servomotor drive system 800.As described in more detail below, medial manifold 400 and distalmanifold 500 included bearings and seals to stabilize rotation andprevent fluid entry into device 10. Manifolds 400, 500 and 600 werestationary during operation, thereby providing a base to attach support300 and HFM bundle 200. Flow through the device was driven by a vacuumpump 750 connected to the proximal manifold outlet port 620. Drivesystem 800, including a DC servomotor and an amplifier, provided therequired torque to actuate and control drive line 700.

In the illustrated embodiment, the sweep gas flowed into (via inlet port610) and through a flexible pneumatic shaft or tube 350 (fabricated, forexample, from a polymer such as polyurethane) and into medial manifold400. Drive line 700 passed through a conduit 710 which was positionedwithin tube 350. As illustrated by arrow S in FIG. 5D, inflowing sweepgas flowed in an annular passage between conduit 710 and the inner wallof tube 350. At medial manifold 400, the sweep gas (represented byarrows S in, for example, FIGS. 6C and 6D) was distributed into a firstend of hollow fibers 210 of hollow fiber bundle 200. Upon exiting asecond end of hollow fibers 210, the sweep gas was collected withindistal manifold 500. In distal manifold 500, the returning sweep gas(represented by arrows R) entered a fluid path within a connector 510rotatably positioned within distal manifold 500. Returning sweep gaspassed from connector 510 into a central fluid path within impellershaft 120, with which connector 510 was in operative and fluidconnection.

The returning sweep gas passed through the center of device 10 (withinthe central fluid path within impeller shaft 120) back to medialmanifold 400 as, for example, illustrated in FIGS. 6C and 6D. In thatregard, impeller shaft 120 was in operative and fluid connection with adistal end of a connector 410 rotatably positioned within medialmanifold 400. Drive line 700 was in operative connection with a proximalend of connector 410 and imparted rotational motion to connector 710 andthereby to impeller shaft 120. From medial manifold 400, returning sweepgas passed through an annular space between drive line 700 and aninterior wall of conduit 710 to proximal manifold 600. The returningsweep gas exited device 10 by flowing through conduit 710 to outlet 620of proximal manifold 600 as, for example, illustrated in FIG. 5D.

Once again, a negative pressure gradient (created, for example, via asealed vacuum) was used to drive oxygen flow from inlet port 610 toexhaust gas outlet 620. The sweep gas flowrate requirement for aprototype exchanging 75 ml CO₂/min was determined to be at least 3 LPMusing the 40-60 multiplier discussed above. The main resistance to flowin device 10 was the HFM bundle and the resistance was estimatedaccording to the following equation assuming oxygen to be a compressiblegas:

$\begin{matrix}{{P_{o}^{2} - P_{L}^{2}} = {2\left( \frac{128\mspace{14mu} \mu \; L}{N\; \pi \; d^{4}} \right)P_{atm}Q^{RTP}}} & (1)\end{matrix}$

where P_(o) is upstream pressure, P_(L) is downstream pressure, t is gasviscosity, L is fiber length, N is total number of fibers, d is fiberinner diameter, P_(atm) is atmospheric pressure, and Q^(RTP) is sweepgas flowrate (LPM). For a flowrate of, for example, 3.5 LPM, thecorresponding pressure drop over the bundle was approximately 50 mmHg.See Eash H J, Frankowski B J, Hattler B G, Federspiel W J: Evaluation oflocal gas exchange in a pulsating respiratory support catheter. Asaio J51: 152-157, 2005, the disclosure of which is incorporated herein byreference. Vacuum pump 750 was capable of producing 9 LPM flow at thispressure head, and therefore the bundle design was adequate to attainsufficient sweep flow.

High-speed electromechanical drive system 800 included a brushlessDC-servomotor (Series 2444-024B, MicroMo Electronics Inc. of Clearwater,Fla.) and 2-quadrant PWM servo amplifier (Series BLD 3502, MicroMoElectronics Inc.) and provided rotation and control. A conditionalanalog voltage of 0-5 volts set by a potentiometer dictated motor speed.The unit was powered by a 24 volt AC/DC quad output power supply (SeriesNFS110-7602P, Artesyn Technologies, Inc. of Boca Raton, Fla.). Thecomponents were housed within a single box for ease of use andprotection from electrical failure. Drive system 800 included a mini DCcooling fan to remove heat from the power supply. Motor speed wasmeasured with a digital rate indicator (Model CUBS, Red Lion Controls ofYork, Pa.). A wire connection was introduced to the amplifier to tapvoltage pulses returning from the servomotor hall-sensors. Connectionfrom the amplifier to the digital rate indicator enabled pulses to becounted and displayed. A mechanism for measuring motor torque was alsoincluded in the setup. The torque generated by the servomotor wasdirectly proportional to the current requirement for motor operation. Awire connection was made to directly measure the DC current output frompower supply to amplifier. The measurement was representative of thesupplied current from amplifier to motor and was used to calculate motortorque by the servomotor torque constant. Finally, a fuse was placed inseries from power supply to amplifier. The fuse automated the system toshutdown upon motor stall and protected the motor from damaging currentlevels.

As discussed above, each of medial manifold 400 and distal manifold 500included a seal and a bearing to, for example, prevent blood intake andto support concentric impeller rotation. A diamond like carbon coating(DLC) can, for example, be employed where the rotating shaft and theseals make contact to, for example, reduce the coefficient of frictionand add lubricity. Medial manifold 400 is illustrated in, for example,FIGS. 6A through 6D. A rotary seal 404 manufactured from nitrile rubber(#711800, Chicago Rawhide/SKF USA, Inc. of Elgin, Ill.) was used in thestudied embodiments. Seal 404 accommodated shaft diameters of0.110″-0.140″ and had an outer diameter of 0.313 inches. Seal 404 wasmodified to an outer diameter of 0.250 inches to meet sizespecifications. Seal 404 was also configured with a shielded, mini ballbearing 406 (#35380815, MSC Industrial Supply Co. of Melville, N.Y.)possessing a 0.125 inches shaft diameter and 0.250 inch outer diameter.In several studied embodiments, a generally cylindrical sleeve ofmaterial 408 (such as VESPEL®, a polyamide based polymeric materialavailable from DuPont) was positioned within the housing of medialmanifold 400 adjacent to the distal or blood-side face of seal 404.Material 408 was 0.5 inches in length and had an inner diameters of0.113 inches. That diameter created a 0.0005 inch clearance between aconnector section 122 of impeller shaft 120 and material 408.

Distal manifold 500 is illustrated in FIGS. 7A and 7B. As with medialmanifold 400, a rotary seal 504 fabricated from nitrile rubber (#711800,Chicago Rawhide/SKF USA, Inc.) was used. Seal 504 accommodated shaftdiameters of 0.110″-0.140″ and had an outer diameter of 0.313″. Seal 504was modified to an outer diameter of 0.250 inches to meet sizespecifications. Seal 504 was also configured with a shielded, mini ballbearing 506 (#35380815, MSC Industrial Supply Co.) having a 0.125 inchshaft diameter and 0.250 inches outer diameter. As described above inconnection with generally cylindrical sleeve of material 408, agenerally cylindrical sleeve of material 508 (VESPEL) was positionedwithin the housing of distal manifold 500 adjacent to the proximal orblood-side face of seal 504. As was also the case with material 408,material 508 was 0.5 inches in length and had an inner diameters of0.113 inches. The diameter created a 0.0005 inch clearance between aconnector section 124 of impeller shaft 120 and material 508.

In several studied embodiments of device 10, the selected material forflexible impeller shaft 120 was wire reinforced PEBAX® (a polyetherblock amide available from Arkema France Corporation of Colombes,France) catheter tubing (#N98-TOL-664, available from New England WireTechnologies of Lisbon, N.H.). The tubing was chosen from severalsamples based on tortuous loop test results discussed in Appendix A.Impeller shaft 120 was operatively connected to drive shaft or driveline 700 via connector 410 as described above, which was produced froman internal component of bi-directional 0.072 inch diameter shaft(#067N133C, S.S. White Technologies of Piscataway, N.J.) that wascoupled to servomotor system 800 via proximal manifold 600 as alsodescribed above.

Tortuous loop tests described in the Experimental Section hereofdemonstrated that 0.5 inch length impeller units 100 with 0.5 inchesspacing therebetween upon flexible impeller shaft 120 were suitable toattain desired impeller flexibility. Moreover, the spacing betweenimpeller units 100 can promote recirculating flow from impeller segmentsinto nonimpeller regions and thereby promote gas exchange. Variousimpeller/vane units 100 evaluated in device are shown in FIGS. 8A(1)through 8A(9). Enlarged illustrations of a sawtooth impeller unit 100 ofthe present invention are set forth in FIGS. 8B and 8C. A cylindricalblank was also made as a control and is not displayed. Paddle and axialimpeller units were fabricated in-house. Axial units were produced byturning static mixers (#04667-18, Cole Parmer Instrument Co. of VernonHills, Ill.) on a lathe to a 0.152″ outer diameter. Other impeller units100 were produced by SLA rapid prototyping. 2-flat, 4-flat, sawtooth,paddle, centrifugal and cone were developed as radial units. The RAXvanes included a combination of radial and axial geometries to assist incarrying fluid to the radial blades of the unit. All studied impellerunits 100 had inner diameters of 0.084 inches, outer diameters of 0.156inches, and lengths of 0.5 inches.

WATERSHED™ 11120 stereolithography resin available from DSM Somos ofElgin, Ill. was used for impeller unit manufacture. The material wascompatible with SLA prototyping and also resistant to wateradsorption/swelling. Limited clearance within the cavity within HFMbundle 200 made water resistance a desirable property. Once again,materials/sleeves 408 and 508 were made from DuPont VESPEL; which hasdemonstrated applicability as a low wear material which absorbs verylittle water. Drive components, medial manifold 400 and distal manifold500 were fabricated from 316 stainless steel on a Hardinge lathe.Proximal manifold 600 was formed from acrylic polymer. Inlet and exhausttubings were made of polyurethane plastic. Component mating wasaccomplished using the 2:3 polycin to vorite epoxy or PERMATEX® clearRTV silicone adhesive sealant (available from Permatex, Inc. ofHartford, Conn.). Mechanical drive parts were connected using silversolder.

Initial evaluations of a number of devices 10 including PEBAX tubing forimpeller shaft 120 in an animal model revealed mechanical failures ofsuch an impeller shaft 120. Prototype dissections also showedaccumulations of paste-like blood constituents in the external (bloodside) materials/sleeves 408 and 508 that were believed to causevariations in friction, ultimately resulting in the tubing failure. Theannular space between connector sections 122 and 124 andmaterial/sleeves 408 and 508, respectively, may, for example, haveundesirably served as a pathway through which blood traveled to contactseals 404 and 504. In that regard, the blood may have penetrated theexternal housing prior to contacting seals 404 and 504. Shear stresscalculations estimated a stress exposure of 5,000 dynes/cm² at 20,000RPM (Appendix B.1). Paul et al. found the shear stress threshold forhemolysis to be 3750 dynes/cm² and 4250 dynes/cm² at exposure times of1.4 and 0.425 seconds respectively. Paul R, Apel J, Klaus S, Schugner F,Schwindke P, Reul H: Shear stress related blood damage in laminarcouette flow. Artif Organs 27: 517-529, 2003, the disclosure of which isincorporated herein by reference. The shear stress level was thuspossibly causing hemolysis and contributing to the accumulation of bloodconstituents.

The PEBAX tubing also experienced non-uniform rotation between medialmanifold 400 and distal manifold 500 that created torsional stressesalong the tubing length. The combination of high-speed and torque causedthe PEBAX tubing to fail in two ways: either the tubing completelyruptured leaving a helical pattern in the direction of motor rotation,or the hollow tubing collapsed on itself with the same observed pattern.The pattern indicated a delay or complete seizure in impeller angularvelocity in the distal manifold causing the tubing to overturn.

Given the failure of PEBAX tubing for impeller shaft 120 in severalstudies, a concurrent effort was undertaken to prevent blood thrombosisand improve impeller tubing strength. A flushing fluid (for example,heparinized saline) purge of seals 404 and 504 and exterior (blood-side)annular spaces between connector sections 122 and 124 andmaterials/sleeves 408 and 508, respectively, was implemented withinmedial manifold 400 and distal manifold 500, respectively, to preventthrombus and blood damage. The purge benefited device 10 three-fold by:(1) impeding blood penetration into the annular spaces adjacentconnector sections 122 and 124, (2) preventing direct blood contact withseals 404 and 504, and (3) removing friction-generated heat from seals404 and 504.

To implement a saline purge, orifices were drilled into medial manifold400 and distal manifold 500 to deliver the saline to the exterior faceof the seals 404 and 504 and flush through the annuli betweenmaterials/sleeves 408 and 505 and connector sections 122 and 124,respectively. As, for example, illustrated in FIGS. 5D and 6D, two 304stainless steel hypo tubing lines 900 and 910 (#HTX-25R-24, Small PartsInc. of Logansport, Ind.) with IDs of 0.010″ and ODs of 0.020″ werepassed through tubing 350 to enter the annular path between conduit 710and tubing 350. Alternatively, saline (and/or other flushing fluid(s))can enter device 10 via a saline port 630 formed in proximal manifold600 (see FIG. 4), which can, for example. be in fluid connection withone or more delivery conduits such as hypo tubing lines 900 and 910. Inthe studied embodiments, hypo tubing line 900 connected to medialmanifold 400 to deliver saline. Hypo tubing line 910 passed throughmedial manifold 400 and along the exterior of HFM bundle 200 to connectto proximal manifold 500. Sufficient saline pressure (for example, atleast 200 mmHg) was used to prevent blood seepage. Using a pressure of200 mmHg as a guideline, the flowrate of saline through a single annularspace to generate the pressure standard was calculated to be 12 ml/hr(see Experimental Section (B.2.)). A minimal saline flowrate of 12 ml/hrwas used in all studies.

A controlled volume of flushing fluid or liquid (saline/heparin) wasthus introduced into an engineered space (the annular space betweenmaterials/sleeves 408 and 508 and connector sections 122 and 124,respectively) between the blood and seals 404 and 504. As discussedabove, this space was approximately 0.5 Inches long, and there was anapproximately 0.0005 inch gap between the sleeves/seal housings and therotating impeller shaft connector sections. The flushing fluid enteredinto the bearing seal housing of the manifolds on the external side (orblood side) of seals 404 and 504 and traveled through the engineered gapwherein it purged/prevented inflow of blood. This space acted as abarrier between the blood and the seals and rotating components. Theflushing fluid purge minimized or eliminated contact between blood androtating components that cause thrombus and blood damage by heat orfriction generated at the interface of a rotating shaft and the seal.The flushing fluid/saline also provided lubrication and cooling to theshaft-seal interface.

To improve impeller shaft strength, a stainless steel coil(#HVT18642-01, available from Heraues Vadnais Inc. of Saint Paul, Minn.)was used in several studies for impeller shaft/tubing 120. The coilincluded 4-filar units that served to individually distribute torqueloads. The coil was made to dimensions of 0.060″ ID and 0.083″ OD from0.010″ 304 stainless steel wire wound in a clockwise direction. Thisembodiment of impeller tubing 120 was formed as a hollow flexible driveshaft and could transmit higher loads than plastic tubing. PEBAX heatshrink tubing (#P2-140-006-CLR, available from Cobalt Polymers ofCloverdale, Calif.) with a wall thickness of 0.006″ was applied to thecoil to produce the finalized, sealed tubing used in devices 10.Impeller shaft 120 was tested in a torsion testing apparatus. Impellershafts 120 formed from stainless steel coil achieved much largertorsional deflections prior to failure than the previously studied PEBAXtubing and were integrated into device 10 for further studies. Althoughthe stainless steel coil embodiment of impeller shaft 120 exhibitedgreater strength than the PEBAX tubing embodiment, shear stresses weredecreased sufficiently via use of a flushing fluid purge as describedabove, that the PEBAX tubing embodiment of impeller shaft 120 could havebeen implemented without failure when used in connection with such apurge.

As described above, various impeller units 100 geometries were evaluatedin bench studies, which were followed by studies in a bovine model. Gasexchange tests of devices 10 were conducted in vitro to assess gasexchange efficiencies with the various impeller units 100. Devices 10were tested in the standard DI water and a viscous solution similar toblood to provide insight regarding impeller geometry performance.

The in vitro test was an idealized simulation of the vena cavaenvironment and provided rapid device characterizations allowing deviceto device comparisons and indication of trends that could be observed invivo. Testing was performed by introducing the device into separatefluid and gas circuits. The fluid loop re-circulated de-ionized waterpast the device. Water was thus the blood representative fluid andserved as the gas carrying medium. Device 10 was placed in the fluidcircuit within a 7/8″ diameter acrylic test section that simulated thevena cava. The fluid circuit contained a reservoir, fluid pump,thermometer, heater, flow meter, pressure transducer and de-oxygenator,and all were connected with standard laboratory TYGON® tubing (plastictubing) available from Saint-Gobain Performance Plastics Corporation ofAurora, Ohio. A schematic of the test setup is illustrated in FIG. 9A.

The de-oxygenator was used to establish test section water inletconditions. Conditions set were a pO₂ of 25-35 mmHg and a pCO₂ of 47-53mmHg, which represented physiological normocapnia venous blood. Thewater conditions were monitored on an ABL 555 soluble gas analyzermachine. Water flowrate through the loop was maintained at 3 LPM with atemperature of 37° C., and the water gauge pressure within the testsection was maintained at approximately 10 mmHg. No micro-bubbleformation was observed.

The gas circuit established the pathway for oxygen sweep gasintroduction and exhaust gas analysis. Device inlet 610 was connected toa pure oxygen source maintained at atmospheric pressure. Upon exitingdevice 10 via outlet 620 the sweep gas was passed through a coldmoisture trap for dehumidification and then directed through a flowmeter and CO₂ analyzer. The flowrate was driven by a vacuum pressurestation that pulled sweep gas through the entire circuit. Gas pressuredrop over device inlet 610 to outlet 620 was monitored using a pressuretransducer.

In gathering gas exchange data, water conditions were first measured toensure that inlet pCO₂ and pO₂ were within desired range. Acceptance ofinlet conditions was followed by recording of water temperature,flowrate, and pressure in the fluid circuit. Also, the sweep gasflowrate, device pressure drop, motor torque, and exhaust CO₂concentration were recorded. The RPM rate was then changed and theprocedure was repeated to obtain a minimum of two random data points foreach RPM to ensure repeatability.

CO₂ removal, V_(CO2) was calculated for the rates by the followingequation:

V _(CO2) =Q ^(STP) F _(CO2)  (2)

where Q^(STP) is the flowrate of sweep gas at standard temperature andpressure and F_(CO2) is the fraction of CO₂ in the exhaust of thecatheter, as measured per the CO₂ analyzer. The exchange rate was thendivided by the HFM surface area and normalized to the target pCO₂ inletof 50 mmHg to calculate efficiency (VCO₂) and normalize variabilityassociated with small changes in inlet pCO₂ concentrations:

$\begin{matrix}{{VCO}_{2} = {V_{{CO}\; 2}\frac{50}{{p{CO}}_{2}^{INLET} \cdot {SA}}}} & (3)\end{matrix}$

Gas exchange testing evaluated the effect of impeller geometry on CO₂removal efficiency and O₂ exchange efficiency. CO₂ removal efficiency indeionized water for four different impeller geometries as a function ofimpeller rotation rate is illustrated in FIG. 9C. O₂ exchange efficiencyin deionized water for four different impeller geometries as a functionof impeller rotation rate is illustrated in FIG. 9D. As described above,in addition to testing in deionized water, CO₂ removal efficiency wasalso measured in a viscous solution. The viscous solution test evaluatedthe interplay between the impeller transmitting motion to the fluidversus the bundle impeding fluid velocity. A higher viscosity solution,such as blood, transmits fluid motion better thus increasing relativevelocity to fiber, whereas the bundle region impedes fluid motion anddecreases relative velocity. A 0.4% w/w concentration ofcarboxymethylcellulose (CMC) sodium salt (#C5678-500G, Sigma-AldrichInc.) and DI water was interchanged with homogenous DI water to testprototypes in both solutions. The viscosity of the CMC solution wasmeasured at 37° C. during each test using a capillary viscometer (#150,Cannon Instrument Co) to ensure that the viscosity remained in a rangeof 2.5-2.7 cP. CMC was used as the solution thickener because a minuteconcentration could be added to water without altering the gassolubility properties.

Maximum gas exchange in the studies occurred at the peak rotational rateof 20,000 RPM. FIGS. 9E(1) through 9E(9) display prototypecharacterization plots of VCO₂ versus RPM in both testing solutions forthe nine impeller unit geometries testes. Maximum CO₂ removalefficiencies for all geometries are listed in Table 2. The 4-flat, RAX,sawtooth, and 2-flat geometries performed similarly. Without limitationto any mechanism, it was hypothesized that the centrifugal geometry hada lower efficiency compared to other radial geometries since the endswere enclosed and fluid could not penetrate the vanes as well asopen-ended geometries. The cone geometry was the worst performing unit,even when compared to the cylindrical control. Without limitation to anymechanism, it was hypothesized that the tapered end caused the cone unitto have a lower average linear velocity at the surface, whichtransmitted less momentum to fluid particles, causing the cone geometryto underperform even the control.

TABLE 2 VCO₂ Impeller (ml CO₂/min/m²) control 307 ± 3  4-flat 529 ± 21RAX 514 ± 11 sawtooth 482 ± 6  2-flat 482 ± 3  paddle 457 ± 1  axial 446± 3  centrifugal 341 ± 2  cone 264 ± 10

The effect of increased viscosity was consistent between geometries. Afluid viscosity in the range of 2.5-2.7 cP decreased gas exchange byapproximately 20-30% at 20,000 RPM when compared to water. Withoutlimitation to any mechanism, it is believed that the reduction was aresult of increased drag forces on fluid particles as they traversed theHFM bundle. Velocity of a fluid flowing through a porous medium can betheorized according to Darcy's Law which states:

$\begin{matrix}{{\overset{\rightharpoonup}{V}o} = {{- \frac{k}{\mu}}\left( {{\nabla\; P} - {\rho \; \overset{\rightharpoonup}{g}}} \right)}} & (4)\end{matrix}$

where {right arrow over (V)}o is the velocity field vector, k ispermeability, μ is viscosity, ∇P is the pressure gradient, ρ is fluiddensity, and {right arrow over (g)} is the force of gravity. The fluidvelocity through the medium is indirectly proportional to the viscosityand fluids with higher viscosities exhibit lower relative velocities tofibers at a given RPM rate, thus inhibiting boundary layer reduction.This effect was observed in comparing results obtained for the twosolutions, per the reduction in efficiency.

CO₂ exchange in blood was approximately 20-40% greater than in waterwhen compared, for example, to the intravascular gas exchange devicedescribed in Federspiel W J, Hewitt T, Hout M S, et al: Recent progressin engineering the Pittsburgh intravenous membrane oxygenator. Asaio J42: M435-442, 1996, referred to as the HATTLER™ Catheter. Oxygenexchange was also significantly greater.

The blood viscosity resembling solution provided a reference forestimating impeller prototype efficiency enhancement in blood byincorporating relevant viscous effects into the system. The expectedincrease in efficiency as a result of blood CO₂ capacity was similar tothe magnitude of efficiency reduction caused by viscosity. Therefore, itwas hypothesized that CO₂ removal efficiencies in vivo would achieverates similar to those observed in standard DI water.

A common empirical mass transfer correlation for analyzing oxygenatorgas exchange was determined from an analogous heat transfer correlationfor perpendicular tube bank flow as follows: Sh=aRe^(b)Sc^(1/3), whereinSh is the Sherwood number, Re is the Reynolds number, and Sc is theSchmidt number. The coefficients a, b are dependant on fiber bundlegeometry and are determined experimentally, however, coefficient b canbe loosely approximated as b=0.5. If the velocity term in the Reynoldsnumber is assumed proportional to rotation rate, VCO₂ can be compared toRPM^(1/2) to assess mass transfer. FIGS. 9F(1) through 9F(9) show thecorrelation for the nine impeller geometries tested on the bench. Thelinear relationship between VCO₂ and RPM^(1/2) indicated that the CO₂driving gradient for gas exchange at higher RPM was not reduced, andtherefore was not likely an explanation for the observed leveling trendof gas exchange. The trend was not investigated further. However,without limitation to any mechanism, a second hypothesis to explain theplateau was that specific fiber layers or fiber regions within thebundle were not fully utilized in exchanging gas as result of aninefficient fluid mixing mechanism.

Experiments were also performed to test the effectiveness of theheparinized saline purge by first testing devices of the presentinvention without saline purge. The tests were conducted in a benchblood circuit to identify whether blood infiltration and subsequentaccumulation was responsible for the torsional failures witnessed in theinitial in vivo studies described above. Mock devices including only therotational components were first tested, and results warranted furthertesting with full prototypes. Two devices were tested in the finalfatigue study. The first was a device absent saline purge that served asthe test control; while the second was a device with saline purgesystem.

Both devices were placed in parallel ⅞″ test sections within the bloodcircuit. Blood was re-circulated through each test section at a flowrateof 1.5 LPM and maintained at 37° C. Two liters of blood (Heparinizedbovine blood, Hemostat Laboratories) were used in the circuit. The bloodwas characterized at the beginning for ACT and fibrinogen content. Aschematic of the test system is shown in FIG. 9B.

Saline was pumped into each of medial manifold 400 and distal manifold500 at a flowrate of 12 ml/hr using a peristaltic pump. The driveconsoles were programmed to record elapsed time and 1¼ amp fuses wereinstalled into each. The fuses automated the system to shutdown uponfailure and permitted the fatigue testing to be conducted overnightunder no supervision. The prototypes were operated at a constant speedof 10,000 RPM for the test duration. Motor torque requirements andelapsed time were recorded. All equipment used for fatigue testing isdescribed in Table 4.

TABLE 4 Component Model Manufacturer Blood 2 L heparinized bovineHemostat Laboratories blood Ultrasonic flow T110 Transonic Systems Inc.probe Reservoir bag AFFINITY Reservoir Medtronic Inc. Bag #321 Bloodpump BIO-MEDICUS ® Medtronic Bio-Medicus Inc. biopump BPX-80 Pumpconsole 540 BIO-CONSOLE ® Medtronic Bio-Medicus Inc. Pediatric heatD1078 Medtronic Biotronics exchanger Thermometer DIGI-SENSE ® type TCole-Parmer Instrument Co. thermocouple Temperature pump T/Pump TP-406Gaymar Industries Peristaltic pump MASTERFLEX ® Barnant Company C/L77120-62

Results of preliminary component and full prototype blood fatigue testsshowed the saline purge to be an effective means for preventingtorsional impeller tubing failures. In the full prototype experiment,device 10 with saline purge was operated for 24 hours and did not showindication of potential failure. The test was discontinued because theoperating time far exceeded that necessary to complete an acute bovinestudy. The device without saline purge, however, failed two hours afterthe test start. A plot of motor current requirement versus elapsed timeis shown in FIG. 10. It is observed that the device with saline purgeoperated at a constant torque until test completion (motor torque isproportional to motor current). An increase in torque to failure wasobserved for the device without saline purge as a result of build-up ofblood constituents within the annular space between materials/sleeves408 and 508 and connector sections 122 and 124, respectively.

Blood accumulation was confirmed in the control device (without salinepurge) by dissecting the prototypes and examining rotational components.The observed build-up of blood constituents replicated observations offailed devices earlier studied in vivo. In contrast, observation of thedevice with saline purge revealed seals and material/sleeves 408 and 508free from blood contact. The saline purge was implemented for subsequentin vivo evaluations.

Testing was transitioned from the bench to an in vivo environment toevaluate device performance in a physiological setting. All in vivostudies performed were acute studies with the objective of obtaining CO₂exchange measurements in a bovine model. The three impellers determinedfrom bench gas exchange characterizations to provide the best CO₂efficiency were the 4-flat, RAX, and sawtooth impeller unit designs.Those three impeller unit designs were used in all further in vivotesting. Gas exchange was measured in two different vena cava positions.The first location was a position spanning the right atrium wherelongitudinal blood flow in the IVC and a cross-flow into the rightatrium were experienced. The second position evaluated the cathetersolely in the IVC where only longitudinal flow past the device occurred.

All experiments were performed at a University of Pittsburgh animalfacility possessing the proper staff and surgical equipment to providefull animal care. The surgical procedure and testing protocol wereauthorized by the University of Pittsburgh IACUC. Calves weighingapproximately 100-125 kg were used in the experiments. Each animal wasfitted with a Swan-Ganz catheter and femoral pressure line followinganaesthetization. Heparin infusion was administered into the femoralvein to maintain activated clotting times (ACTs) above 500. Guidewiresfor device insertion were laid through the jugular vein and femoral veinusing a 2-cupped magnet system. The jugular vein was exposed and anincision was made. Upon insertion the device was extended through thejugular vein into the vena cava to situate in either of the twolocations described above.

All equipment used in the gas circuit for bench exchangecharacterization was used. CO₂ removal efficiency was calculated inaccordance to equations (2) and (3). The purge flowrate was 12 ml/hr of1:50 heparin to saline solution. The femoral blood pCO₂ was maintainedin the range of 45-55 mmHg by adjusting ventilator settings.

Testing protocol began when calf femoral pCO₂ stabilized within therequested range. The prototype was tested over the full RPM range of200-20,000 RPM in a randomized fashion. CO₂ exhaust concentration, sweepgas flowrate, sweep gas pressure drop, and torque were acquired at ratesof 200, 5,000, 10,000, 15,000 and 20,000 RPM. Each data point wasobtained at least twice to ensure repeatability. The protocol wasinitiated in both vena cava locations before a different impellerprototype was inserted. Calve arterial and femoral pressures weremonitored and plasma free hemoglobin samples were obtained through thecourse of testing. Following the experiment, a necropsy was performed toview the vasculature. Prototypes were later dissected on the bench toobserve any thrombus formation, failure sites, and seal wear/appearance.A concise outline of the protocol is set forth in Appendix C.

Impeller catheter gas exchange data was obtained in four differentcalves. The results of impeller geometry influence matched the trendidentified in bench-testing. FIG. 11 shows the maximum average CO₂removal efficiencies for 4-flat, RAX, and sawtooth impellers. The 4-flatgeometry produced the most favorable efficiency level and was found tobe significantly higher than the sawtooth geometry. The corresponding invivo removal rates were: 36 ml CO₂/min for 4-flat, 34 ml CO₂/min forRAX, and 33 ml CO₂/min for sawtooth.

A typical plot of efficiency versus RPM is displayed in FIG. 12. Gasexchange increased as a function of rotational rate. The slope of thecurve, however, began to trend towards zero near 20,000 RPM indicating amaximum efficiency plateau. An inspection of all data did not showdevice location spanning the right atrium to be more beneficial incomparison to the IVC. Any conclusion from the above observations may belimited by the necessity to configure the prototype in vena cavapositions avoiding severe tortuousness which may have exposed thecatheter to the right atrium mixing in both intended locations.

Maximum removal efficiencies in blood were very similar to bench gasexchange results obtained using normal DI water. Without limitation toany mechanism, it was hypothesized that an increase in CO₂ removal overDI water was not observed in blood because the ability of blood to carrya higher CO₂ content was offset by the increased bundle drag forces andconsequent reduction in relative fluid velocity resulting from a higherfluid viscosity. Bench-testing of devices 10 in standard DI water shouldthus provide an accurate prediction of maximum CO₂ exchange. Oxygenexchange in blood are observed to be 2-3 time greater than that observedin deionized water.

Once again, plasma free hemoglobin (PFHB) samples were measured duringtesting. At least three devices 10 were inserted per calf. Data from twostudies in which a specific impeller was evaluated over three hours isshown in FIG. 13. FIG. 17 illustrates a plot of PFHB versus elapsed timefor all experiments. The prototypes appeared to generate consistentlevels between 20-40 mg PFHB/dL that trend towards a constant ordecreasing rate. Although devices 10 and impeller units 100 were notoptimized, undesirable levels of hemolysis were not observed duringtesting.

The non-optimized impeller catheters of the present invention enhancedin vivo CO₂ removal efficiency by 70% over, for example, the HATTLERCatheter and confirmed the effectiveness of rotation. The device wascapable of operating in tortuous vasculature while protecting the venacava wall. The tradeoff in HFM surface area to reduce size however,reduced overall CO₂ removal rates. Removal rate was ultimately higher inthe HATTLER Catheter because of its 2½ fold greater surface area. Table4 sets forth an overview of catheter performance for the HATTLERcatheter and impeller device 10 of the present invention.

TABLE 4 Surface In vitro In vivo CO₂ Removal Area Efficiency EfficiencyRate Catheter (m²) (ml CO₂/min/m²) (ml CO₂/min/m²) (ml CO₂/min) HATTLER0.17 230 300 51 Catheter Impeller 0.07 529 513 36

In the devices, systems and methods of the present invention, CO₂removal increases in proportion to CO₂ tension in blood. Respiratorycatheter data from acute animal implants (calves and sheep) in whichventilatory challenges were done to vary the venous PCO₂ were regressed.The resulting correlation (r²=0.993) from 25 mmHg to 70 mmHg: {dot over(V)}_(CO2){dot over (V)}_(CO2) ^(50mmHg)=1.32(10⁻⁴)P_(CO2)²+8.05(10⁻³)P_(CO2)+0.26 predicts an increase in the CO₂ removal rate of50% from normocapnia (45-50 mmHg) to permissive hypercapnia (˜70 mmHg).Thus, removal rate of CO₂ can be increased approximately 50% byoperation under conditions of permissive hypercapnia.

Immobilizing carbonic anhydrase enzyme onto the hollow fiber membranesof the devices of the present invention as described in U.S. patentapplication Ser. No. 11/811,265 and PCT International Patent ApplicationNo. PCT/US2007/013563 (PCT Publication No. WO 2007/146162), thedisclosure of which are incorporated herein by reference, furtherincreases CO₂ efficiency. Preliminary data using current (baseline)immobilization of CA on hollow fibers (activity ˜0.002 U/cm²) indicateda 25% increase in the CO₂ removal rate in blood. Improved immobilizationmethods have increased CA activity by 250%.

Agitation mechanisms such as rotating impellers increase gas exchange byreducing the size of diffusional boundary layers that dictate gasexchange Immobilized CA increases CO₂ exchange by creating facilitateddiffusion of CO₂ as bicarbonate ion, resulting in a larger effectivediffusion coefficient for CO₂. As such, the effects would be additive.However, such effects may be more than additive because as the boundarylayer is reduced, so is the amount of native CA (i.e. in red cells)within the boundary layer, with which immobilized CA competes.Preliminary studies indicate that without native CA (i.e. usingbicarbonate buffer as the test fluid), the same immobilized CA fibersincreased the CO₂ removal rate by 75%.

As set forth above, carbonic anhydrase catalyzes blood bicarbonate intocarbon dioxide and increases the fundamental CO₂ exchange drivinggradient. In that regard, carbon dioxide is present in blood in threeprimary forms: CO₂ (dissolved), bicarbonate (HCO₃ ⁻), or carbamate. Asknown in the chemical arts, CO₂ is interconvertible among these formsand the various forms can be in equilibrium with each other as describedby a CO₂ dissociation curve. Most of the CO₂ in blood, however, existsin the form of HCO₃ ⁻ in plasma and in red blood cells. Colton C K.1976. Fundamentals of gas transport in blood. In: Zapol W M and Qvist J,editor. Artificial lungs for acute respiratory failure. Washington D.C.:Hemisphere Publishing Corporation. p 3-43. In that regard, approximately94% of plasma CO₂ and 82% of red blood cell CO₂ is in the form of HCO₃⁻. The two species are interconvertible via the reaction:

CO₂+H₂O⇄H⁺+HCO₃ ⁻

The CO₂ generates via metabolic pathways in tissue and diffuses into redblood cells (RBCs), where it is hydrated into HCO₃ ⁻ and hydrogen ions(H⁺) by intracellular carbonic anhydrase (CA). The hydrogen ions formedare bound to hemoglobin while HCO₃ ⁻ is diffused into plasma. However,very little CO₂ is hydrated in plasma because of a lack of CA in plasma.In lungs, the reaction is reversed. HCO₃ ⁻ is converted into CO₂ via CAin red blood cells, and then exhaled. Some CA exists in lung tissue.

With reference to FIG. 14A, the immobilized enzyme facilitates diffusionof the specific CO₂ through the membrane by generating diffusion of HCO₃to the surface of the membrane. At the membrane surface the enzymeconverts HCO₃ to CO₂.

CA maintains substantial activity upon immobilization (for example, viacovalent bonding to a polymeric material). As set forth in FIG. 14B,radio frequency glow discharge (RFGD) and cyanogen bromide (CNBr)activation chemistry can, for example, be used to immobilize CA on (orin the vicinity of)at least a first or outer surface of the fibers ofthe HFM via covalent bonding as described in U.S. patent applicationSer. No. 11/811,265 and PCT International Patent Application No.PCT/US2007/013563 (PCT Publication No. WO 2007/146162).

The activity of immobilized CA on fiber surfaces can, for example, beincreased by incorporating a base layer of poly(L-lysine) (PLL) ordendritic polyamidoamine (PAMAM) on hollow fibers prior to immobilizingCA. Successive “layers” of CA can be added onto the first immobilizedlayer using glutaraldehyde-linked “layering”.

EXPERIMENTAL SECTION

A. Tortuous Loop Component Tests.

The following provides a brief discussion of tests and results ofcomponent function in a tortuous environment. The tests were alsobeneficial for determining a length of impeller units suitable to retainflexibility. Three impeller shaft tubings were tested in the tortuousloop setup to evaluate torque requirements, fatigue, and impellerlength. The sample group is set forth in Table A-1 below.

The testing device was placed in a loop and water was circulated toprovide a fluid environment. The test device was a “skeleton” impellerprototype in which tubing samples could be easily substituted. Testingwas performed without a HFM bundle except in one instance to evaluatethe effect of the bundle decreasing bending radii. Torque was calculatedby measuring the current into the drive system and converting by meansof the motor torque constant.

TABLE 0-1 Selected tubings for tortuous testing. Sealant ManufacturerPart # Material Mesh Topcoat (A) New NEC00029 Clear PEBAX 40 AWG NAEngland Wire 6333 304 stainless Technologies steel (B) New N98-TOL- Blue44 AWG NA England Wire 664 PEBAX 304 stainless Technologies braid (C)S.S. White 067N133C Stainless NA In-house Bi- directional Steelpolyurethane hollow shaft coil

The final results of all testing indicated that candidate B of Table A-1was a preferred candidate. In that regard, the tubing could withstandrotation at 20,000 RPM in the tortuous loop and also provided thelargest inner diameter to reduce resistance against sweep gas flow.

A.1. Effect of Fluid Viscosity on Torque.

Testing was performed in a straight test section to evaluate whetherfluid viscosity would have a noticeable effect on torque transmission.This was done to ensure that testing in water was suitable and that anincrease in viscosity to that of blood would not add excessive torque tothe motor or tubing that should be considered. The test was performedusing tubing sample A in both water and air. FIG. 15A shows that fluidviscosity had a negligible effect in our applicable ranges.

A.2. Impeller Unit Length.

All impeller units were fabricated from rigid materials and thusconstrained tubing flexibility. Fixing the units to the impeller tubingrequired both a unit length and spacing to be determined. The spacingbetween units was arbitrarily set at 0.5″ (inches). Sample C was testedin the loop with two impeller configurations consisting of either 1″impeller units or 0.5″ impeller units. The test was conducted at 20,000RPM and 1″ units were concluded to provide inadequate flexibility. Thelonger units constrained the tubing to greater bending stresses inbetween units. A portion of the testing configuration is represented inFIG. 15B.

A.3. Fatigue and Torque Requirement.

Test devices were placed in the tortuous configuration represented inFIG. 15B and operated at 20,000 RPM for fatigue life. Elapsed time andmotor torque were monitored. A HFM bundle was also added to theN98-TOL-664 tubing test device to observe the effects on torque asbending radii decreased. Torque versus RPM results for tubing samples Aand B are shown in FIG. 15C. Both samples ran for over 6 hours withoutfailure and were deemed useable in the prototype. The samples performedsimilarly in regards to torque requirement in the loop and the HFMbundle was observed to increase motor torque by an insignificant 0.5-1mN-m. The recommended continuous torque application for the brushlessservomotor was 13.5 mN-m and thus torque required in the “worst-case”scenario tortuous loop was still 3 times lower indicatingmotor-prototype compatibility.

B. Impeller Catheter Design Calculations.

The blood shear stress calculations indicated a plausible cause forbuild-up observed in the external VESPEL sleeves during initial in vivotesting. A flushing fluid (saline) purge calculation was used to meet adesign requirement of 200 mmHg gauge pressure at the seal interface.

B.1. Blood Shear Stress Calculation.

Shear stress calculation for fully-developed, steady flow betweenconcentric cylinders with outer cylinder fixed (ω₂=0) (as graphicallyrepresented in FIG. 16A):

Laminar Flow (Re<1700) between Rotating Cylinders when ω₂=0:

${Re} = {{\omega_{1}R_{1}\frac{\delta}{v}\mspace{14mu} {where}\mspace{14mu} \delta} = {R_{2} - R_{1}}}$

Shear stress τ₁, on the inner cylinder is:

$\tau_{r\; \theta} = {{- \left\lbrack {\mu \; r\frac{}{r}\left( \frac{V_{\theta}}{r} \right)} \right\rbrack_{r = R_{1}}}\mspace{14mu} {where}}$$V_{\theta} = {{{\frac{R_{1}^{2}\omega_{1}}{R_{2}^{2} - R_{1}^{2}}\left( {\frac{R_{2}^{2}}{r} - r} \right)}\mspace{14mu}\therefore\mspace{14mu} \tau_{r\; \theta}} = \frac{2\mu \; R_{2}^{2}\omega_{1}}{R_{2}^{2} - R_{1}^{1}}}$

Calculation for Impeller Rotation at 20,000 RPM:

$\omega_{1} = {{20,{000 \cdot \frac{2\pi \mspace{14mu} {rad}}{60\mspace{14mu} s}}} = {2094\frac{rad}{s}}}$μ_(blood) = 0.033 P$v_{blood}0.031\mspace{14mu} \frac{{cm}^{2}}{s}$ R₁ = 0.142  cmR₂ = 0.144  cm

At 20,000 RPM,

Re=19.2<1700∴flow is laminar

$\tau_{r\; \theta} = {5010\frac{dyne}{{cm}^{2}}}$

B.2. Saline Purge Flow Rate Calculation.

Flow through an annulus (see FIG. 16B) is calculated according to:

$Q = {\frac{\pi}{8\mspace{11mu} \mu}{\left( {- \frac{P}{x}} \right)\left\lbrack {a^{4} - b^{4} - \frac{\left( {a^{2} - b^{2}} \right)^{2}\;}{\ln \left( {a/b} \right)}} \right\rbrack}}$

Assume linear pressure gradient:

${- \frac{P}{x}} = \frac{P_{2} - P_{1}}{L}$

P₁=26,664 Pa P₂=1,333 Pa μ=0.001 Pa−s L=0.0127 m

a=0.001435 mb=0.001422 m

$Q = {{3.28 \times 10^{- 9}\frac{m^{3}}{s}} = {12\frac{ml}{hr}}}$

C. Impeller Prototype In Vivo Protocol.

C1. Impeller Catheter Acute In vivo Test Protocol.

In several studies the impeller catheter of the present invention wasevaluated in a calf model for gas exchange and hemodynamics at variousrotation rates. The results were compared with gas exchange results toin vitro water data.

Devices:

-   -   1. Impeller: Vadnais coil tubing with PEBAX heat-shrink/4-flat        impeller units/saline purge    -   2. Impeller: Vadnais coil tubing with PEBAX heat-shrink/RAX        impeller units/saline purge    -   3. Impeller: Vadnais coil tubing with PEBAX heat-shrink/sawtooth        impeller        -   units/saline purge

Protocol: Pre-Test Preparation (Previous Day):

1) Calibrate all equipment (Gas Flowmeter, Pressure Transducer)2) Setup equipment and make albumin soaking solution and heparinizedsaline (0.02 v/v)

Day of Test:

-   1) Calibrate CO₂ analyzer.-   2) Soak device in albumin/heparin solution. (50 cc bovine albumin,    10 cc heparin, and saline)-   3) Fill moisture trap container with ice.-   4) Take baseline blood samples.-   5) Insert all necessary pressure lines and take baseline    measurements (CO, pressures):    -   Left femoral vein    -   Carotid artery    -   Swan-Ganz—CVP and CO-   6) Measure diameter of right external jugular vein once exposed.-   7) Measure initial hematocrit and CO, then begin volume loading with    lactated ringers at 15 ml/kg. After volume loading, measure    hematocrit and CO again.-   8) The magnetic guide system will be inserted via the right femoral    vein and right external jugular vein, using Fluoroscopy.-   9) Pull wires back and attach distal tip of catheter to femoral    guide wire.-   10) Allow 2 minutes for saline flush of device to initiate.-   11) Administer a heparin bolus through the left femoral line.-   12) Insert the catheter into the right jugular with animal    positioned on its side.-   13) Pull catheter through the jugular vein and position spanning the    RA. Verify with Fluoroscopy.-   14) Set the gas flow to maximum allowed and begin rotating at 5000    RPM. A post-insertion CO and several gas exchange values should be    acquired during the equilibrium phase.-   15) When exchange stabilizes, rotation rate protocol will begin.-   16) Test the following rotation rates in a random order: 5000,    10000, 15000, 20000 RPM, repeating each point twice. Record % CO₂,    rotation rate, current, gas flow rate, and gas pressure drop.-   17) Reduce the rotation rate and record 200 RPM exchange rate.    Obtain a CO at 200 RPM.-   18) When finished with all rotation rates, move the device into the    IVC.-   19) Allow ˜15 minute stabilization period with rotation at 5000 RPM.    Take another CO.-   20) Repeat steps 18-19 in this new location.-   21) Remove device and place additional impeller devices. Repeat    steps 13-21.-   22) Keep the device in the second location for necropsy.-   23) Pictures and anatomic measurements will be taken at necropsy.-   24) Perform gross examination of organs (lung especially) and send    out for histology samples if needed.

C.2. In Vivo Hemolysis Studies.

FIG. 17 illustrates measured PFHB during all in vivo testing of theimpeller prototype. Each line represents a separate calve experiment.PFHB ranged between 20-40 mg/dL for each animal during prototypetesting.

The foregoing description and accompanying drawings set forth thepreferred embodiments of the invention at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope of the invention. The scope of theinvention is indicated by the following claims rather than by theforegoing description. All changes and variations that fall within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1-18. (canceled)
 19. A system for intracorporeal gas exchangecomprising: a rotatable shaft; at least one agitation mechanismpositioned on the rotatable shaft; a plurality of hollow gas permeablefibers adapted to permit diffusion of gas between intracorporeal fluidand an interior of the hollow fibers, the plurality of hollow fibersbeing positioned radially outward from the agitation mechanism; at leasta first seal, and a purge system via which a flushing fluid isintroduced under pressure over a blood-side face of the at least a firstseal and caused to flow through a space on a blood side of the at leasta first seal.
 20. The system of claim 19 wherein the space on the bloodside of the at least a first seal is in fluid connection with an annularspace positioned radially outward from the rotatable shaft.
 21. A bloodcontacting medical system, comprising: at least one seal, and a purgesystem via which a flushing fluid is introduced under pressure over ablood-side face of the at least one seal and caused to flow through aspace on a blood side of the at least one seal.
 22. The system of claim21 further comprising a rotatable member.
 23. The system of claim 22wherein the space on the blood side of the at least one seal is in fluidconnection with an annular space positioned radially outward from therotatable member.
 24. The system of claim 20 wherein the purge systemcomprises a first conduit to deliver the flushing fluid the annularspace.
 25. The system of claim 19 wherein the system further comprises amedial manifold and a distal manifold, a first end of each of theplurality of hollow fibers being in fluid connection with the medialmanifold, a second end of each of the plurality of hollow fibers beingin fluid connection with the distal manifold.
 26. The system of claim 25wherein the medial manifold comprising the at least a first seal and thedistal manifold comprises at least a second seal, the purge systemfurther introducing the flushing fluid under pressure over a blood-sideface of the at least a second seal and causing the flushing fluid toflow through a space on a blood side of the at least a second seal 27.The system of claim 26 wherein the space on the blood side of the atleast a first seal is in fluid connection with an annular space adjacentto a first rotating element operatively connected to the rotatable shaftand the space on the blood side of the at least a second seal is influid connection with an annular space adjacent to a second rotatingelement operatively connected to the rotatable shaft.
 28. The system ofclaim 27 wherein the purge system comprises a first conduit to deliverthe flushing fluid to the medial manifold and a second conduit todeliver the flushing fluid to the distal manifold.